Fins are flexible appendages that extend from the body of a fish or other water animal, such as dolphins and certain whales. They have several functions, including aiding in propulsion and stability. There are two types of fins: median and paired. The median fins are the dorsal, caudal, and anal fins, and the paired fins are the pectorals and the pelvic or ventral fins. Not all fish have the same number or type of fins, such as the American eel, which has one continuous caudal fin that is a merger of the dorsal, caudal, and anal fins.
The median fins are asymmetrical and grow on the fish's back, at the end of its long, muscular tail and at the lower edge of the tail behind the anus. The dorsal fins grow from a fish's back and may be anterior or posterior fins. The anterior fins are located closer to the fish's head, and the posterior are nearer to the tail. A fish may have one to three dorsal fins.
The caudal, or tail, fin may be heterocercal or homocercal. A heterocercal tail has a larger upper lobe, and the vertebral column extends into the upper lobe. A shark's tail is a good example of a heterocercal tail shape. The homocercal tail has two symmetrical or nearly symmetrical lobes.
The anal fins are located on the rear underbelly of the fish, positioned behind the anus and behind the pectoral fins on the sides of the fish. Not all fish have a full set of fins. Some fish that inhabit tight areas like burrows and crevices have lost the dorsal and anal fins through evolution.
The paired fins are similar to the arms and legs in the human anatomy. The pectorals grow just behind the gills and are comparable to a person's arms. The pelvic fins are on the lower part of the body and are similar to a person's legs. Some fish, such as the Atlantic mudskipper, can walk using either the pectoral or pelvic fins.
Fins serve several functions. Typically, the dorsal and anal fins act as keels, and the paired fins function like rudders. Whereas some fish have lost certain fins through evolution, other fish species have developed fins that are more specialized. For example, the flying fish has over-sized dorsal and anal fins that support the fish's body weight during its moments of flight-like soaring. The lionfish has a long, flowing dorsal fin that contains poisonous spines, which protects it from predators.
Besides keeping the fish stable in the water, fins provide a means of propulsion. The tail or caudal fin normally is the fish's power source. The shape of the tail fin can indicate the type of swimmer that the fish is. The fastest swimmers have lunate, or crescent-shaped, tail fins; continuous swimmers have forked tail fins, and the faster swimmers have a deeper fork. The strong, slow swimmers generally have truncated or rounded tail fins.
Other functions of fins include tasting and touching, especially the pectoral fins. The remora fish has a fin on top of its head that acts like a suction cup and allows it to attach itself to larger fish, such as sharks or whales. Through evolution, the ghost pipefish has developed a pouch for carrying its eggs by its belly. The pouch is made of two fins that have become one. Some fish, such as salmon and catfish, have a fatty, limp fin called an adipose fin.